1. Field of the Invention
The present invention relates to an optical disc apparatus and an optical pickup unit.
2. Description of the Related Art
Recently, optical discs (for example, a CD (Compact Disc), a DVD (Digital Versatile Disc)) are prevailing as recording media for recording/reproducing of information. With the prevalence of the optical discs, optical disc apparatuses for recording/reproducing of information to/from the optical discs are prevailing. An optical disc apparatus generally includes an optical pickup unit, an ASP (Analog Signal Processor), a DSP (Digital Signal Processor), a motor driver, etc. The optical pickup unit includes a semiconductor laser for radiating a laser light beam to an optical disc, a photo detector for receiving a reflected light beam of the laser light beam from the optical disc, etc.
The thus configured optical disc apparatus creates signals respectively for various types of servo control based on a photoelectric converted signal that corresponds to the level of the reflected light beam, outputted from the photo detector to radiate accurately the laser light beam to the optical disc. An example of this can be an optical disc apparatus that creates a focusing error signal and a tracking error signal each having a property that a reference DC value is equal to a half of the maximal amplitude value (hereinafter, “S-shaped property to the reference value”, see FIG. 9A) to respectively execute a focusing servo for focusing a laser light beam on a signal layer of an optical disc and a trucking servo for causing the laser light beam to follow a track on the optical disc. The laser light beam is radiated accurately to the optical disc by executing control in the optical axis direction (focusing servo) and control in the radial direction (tracking servo) by driving an actuator of the optical pickup unit based on the focusing error signal and the tracking error signal. Therefore, the photoelectric converted signal outputted from the photo detector is the base for the focusing error signal and the tracking error signal in these types of servo control, and is required to be an accurate signal corresponding to the level of the reflected light beam.
However, the photoelectric converted signal outputted by the photo detector may include dispersion due to property of each photo detector and property of each optical pickup unit. Therefore, a focusing error signal or a tracking error signal or both influenced by the dispersion may be created. For example, as shown in FIG. 9B, a focusing error signal and a tracking error signal each having a property that a difference (offset) is generated between the reference DC value and a half of the maximal amplitude value may be created (variation of the balance of the S-shaped property to the reference DC value, displacement of the level, etc.). In this case, in an conventional optical disc apparatus, there are applied adjusting processes (balance adjustment, level adjustment, etc.) in which the offset generated in the focusing error signal and the tracking error signal is corrected by, for example, superimposing a DC value for correcting the difference between the reference DC value and the half of the maximal amplitude value, in a calculating process executed in the ASP that creates the focusing error signal and the tracking error signal.
The photo detector is mounted being positioned at high precision on an optical housing of the optical pickup unit to output an accurate photoelectric converted signal that corresponds to the radiation state of the laser light beam to the optical disc. Referring to FIG. 10, the positioning of the photo detector will be described below in detail. The photo detector has, for example, four light-receiving faces A to D formed by dividing an entire light-receiving face to receive the reflected light beam from the optical disc. Dividing lines “O” and “P” that divide the entire light-receiving face into the light-receiving faces A to D as above are provided such that the lines O and P crosses each other at a right angle and each form an angle of approximately 45 degrees with the direction of astigmatism generated when an astigmatism method, a differential astigmatism method, etc., are employed for the focusing servo. The direction of the dividing line “P” is provided such that this direction indicates the direction of a tangent line of a track when a push-pull method, a differential push-pull method, a 3-beam method, etc., are employed for the tracking servo.
The photo detector having such four light-receiving faces A to D is mounted on the optical housing being positioned such that, for example, when a laser light beam is radiated accurately to an optical disc, a reflected light beam of the laser light beam is radiated approximately evenly to the light-receiving faces A to D (see a portion (a) of FIG. 10). That is, when the laser light beam is radiated accurately to the optical disc, the photo detector is positioned such that the intersection of the dividing lines O and P and the center of the reflected light beam approximately coincide with each other. An accurate focusing error signal can be created by positioning the photo detector in this manner and executing a calculating process of {(the photoelectric converted signal corresponding to the level of the reflected light beam at the light-receiving face A)+(the photoelectric converted signal corresponding to the level of the reflected light beam at the light-receiving face C)}−{(the photoelectric converted signal corresponding to the level of the reflected light beam at the light-receiving face B)+(the photoelectric converted signal corresponding to the level of the reflected light beam at the light-receiving face D)}. An accurate tracking error signal can be created by executing a calculating process of { (the photoelectric converted signal corresponding to the level of the reflected light beam at the light-receiving face A)+(the photoelectric converted signal corresponding to the level of the reflected light beam at the light-receiving face D)}−{(the photoelectric converted signal corresponding to the level of the reflected light beam at the light-receiving face B)+(the photoelectric converted signal corresponding to the level of the reflected light beam at the light-receiving face C)}.
Such a conventional optical disc apparatus is disclosed in, for example, Japanese Patent Application Laid-Open Publication Nos. 2002-32924 and 2005-71458.
However, in mounting a photo detector on an optical housing, displacement of the photo detector from the position for the photo detector to be positioned may occur due to an error during the mounting, the remaining stress during the mounting, an error of the amount of an adhesive to be applied to fix the photo detector to the housing, etc. Otherwise, though the photo detector can be kept mounted at the position for the photo detector to be positioned for the time being after the mounting, displacement of the photo detector may occur due to variation over time (for example, variation of temperature, variation of humidity, an impact, vibration). Therefore, even though the laser light beam is accurately applied to the optical disc, the reflected light beam from the optical disc may be radiated as, for example, shown in a portion (b) of FIG. 10. That is, the reflected light beam may not be radiated approximately evenly to the four light-receiving faces A to D. The photo detector outputs a photoelectric converted signal in this radiation state and a focusing error signal and a tracking error signal may be created even when the types of servo control such as the focusing servo and the tracking servo are originally not necessary to be executed. Because the focusing servo and the tracking servo are executed respectively based on the focusing error signal and the tracking error signal, the laser light beam is not radiated accurately to the optical disc and recording/reproducing of information to/from the optical disc may be disabled. Otherwise, accurate recording/reproducing of information to/from the optical disc may not be executed.
A means of solving problems described above can be considered to be, for example, a method of executing, in the calculating process described above of the ASP, an adjusting process to the focusing error signal and the tracking error signal created when the displacement of the photo detector occurs. However, depending on the magnitude of the displacement of the photo detector, the displacement may exceed an adjustable range in the ASP. The adjusting process for the displacement of the photo detector has a limit to secure the performance to be originally satisfied by the ASP.